Re: [PATCH] vfio/pci: Don't export DMABUFs for unmappable BARs

Matt Evans
2026-04-16T13:05:34Z

Hi Leon,

On 16/04/2026 09:11, Leon Romanovsky wrote:
> >
> On Wed, Apr 15, 2026 at 11:16:23AM -0700, Matt Evans wrote:
>> Although vfio_pci_core_feature_dma_buf() validates that both requested
>> DMABUF ranges and the PCI resources being referenced are page-aligned,
>> there may be reasons other than alignment that cause a BAR to be
>> unmappable.
>>
>> Add a check for vdev->bar_mmap_supported[index], similar to the VFIO
>> mmap path.
>>
>> Fixes: 5d74781ebc86c ("vfio/pci: Add dma-buf export support for MMIO regions")
>> Signed-off-by: Matt Evans <mattev@meta.com>
>> ---
>> drivers/vfio/pci/vfio_pci_dmabuf.c | 3 +++
>> 1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/vfio_pci_dmabuf.c
>> index f87fd32e4a01..4ccaf3531e02 100644
>> --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
>> +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
>> @@ -249,6 +249,9 @@ int vfio_pci_core_feature_dma_buf(struct vfio_pci_core_device *vdev, u32 flags,
>> if (get_dma_buf.region_index >= VFIO_PCI_ROM_REGION_INDEX)
>> return -ENODEV;
>>
>> + if (!vdev->bar_mmap_supported[get_dma_buf.region_index])
>> + return -EINVAL;
>> +
>
> And it looks like AI has valid concern about this line too.
> https://urldefense.com/v3/__https://sashiko.dev/*/patchset/20260415181623.1021090-1-mattev@meta.com__;Iw!!Bt8RZUm9aw!5DxsN8cDUviPIZqEjG0pZ_VYYbl_RdmWucTGdTZ3ZzlVP_Ysb0n7ykr0eXwFXdpuqvZH2FK3$

Ah, Sashiko has a point, and I think its suggestion of checking lower
down in the default .get_dmabuf_phys (vfio_pci_core_get_dmabuf_phys())
and preserving driver overrides is decent. Will revisit.

To your other question:
> I noticed this check in vfio_pci_core_mmap(). Isn't that sufficient?

The scenario in mind is doing a DMABUF-export for BARs that you haven't
necessarily noticed can't be mmap()ed, and both paths should be checking.

Cheers,

Matt

>
> Thanks
>
>> dma_ranges = memdup_array_user(&arg->dma_ranges, get_dma_buf.nr_ranges,
>> sizeof(*dma_ranges));
>> if (IS_ERR(dma_ranges))
>> --
>> 2.47.3
>>